Abstract This revised proposal describes the mechanism of binding in ex vivo, and in vivo models between small molecules and negatively charged polysaccharides, and applies its findings to the development of new X- ray computed tomography (CT) contrast agents for imaging articular cartilage. Specifically, we expand on our recent reports (J. Am. Chem. Soc., 2009, 131, 2469-2471; Osteoarthritis and Cartilage, 2010, 18, 184- 191; J. Orthopaedic Res., 2011, 29, 704-709; Osteoarthritis and Cartilage, 2011, in press/on line) of using cationic iodinated contrast agents for CT imaging of negatively-charged glycosaminoglycans (GAGs) in articular cartilage. Clinically today, cartilage is not imaged using CT and several research groups are exploring the use of known CT contrast agents, which possess an overall anionic charge, to image GAGs. We hypothesize that the use of a cationic contrast agent will result in a more sensitive technique for imaging cartilage due to its affinity or the negatively-charged GAGs. Importantly, we have preliminary data demonstrating that these cationic iodinated contrast agents bind GAG in a GAG concentration dependent manner and can be used for ex vivo and in vivo imaging of cartilage via X-ray CT. The specific aims of this proposal are: Aim 1: Synthesize a series of cationic, anionic, and neutral iodinated CT contrast agents, Aim 2: Determine the kinetics and binding affinities of the CT contrast agents to GAGs present in ex vivo cartilage tissue, Aim 3: Ascertain the correlation of CT attenuation vs. GAG concentration and develop a quantitative relationship, Aim 4: (A) Perform serial in vivo contrast enhanced computed tomography (CECT) imaging of rabbit knees before and after anterior cruciate ligament (ACL) transection to demonstrate the ability of CECT using cationic contrast agents to measure progressive changes in cartilage GAG compared to direct measurements after sacrifice, (B) Perform pharmacokinetic/toxicity studies in New Zealand White rabbits. Successful completion of these studies will result in: 1) the development of structure-activity relationships and design requirements for highly sensitive cartilage CT imaging agents for quantitative measurements of GAG; 2) imaging of healthy and degraded cartilage in vivo; 3) the pharmacokinetic profile of the contrast agent after administration; and 4) collection of robust data for analysis, discussion, and further hypothesis generation.